Offshore oil rigs are supported by vertical pipes the lower ends of which are embedded in the ocean floor and the upper end of which extends above the surface. When an oil rig is abandoned, the supporting pipes must be sawed so as not to obstruct sea traffic or animal life.
Metal pipe used to support underwater structures come in diameters from the very smallest available up to at least 72 inches. A machine suitable for cutting such pipe must be resistant to deterioration caused by salt water and must be sized to accept and machine the particular diameter of pipe to be cut. Presently, it is the practice to provide a plurality of cutting machines in varying sizes to accommodate the different sizes of pipe. Accordingly, many different sizes of machines must be kept in inventory to deal with a complicated project involving the cutting of numerous sizes of submerged pipe.
One problem that that has plagued machines adapted to cut submerged pipe is that the machines typically include a plurality of contact pads that contact the surface of the pipe, and a pair of moveable arms that compress the pipe against the contact pads. The machine applies a loop of diamond embedded wire against the surface of the pipe and rotates the loop to cut the pipe. Where the pipe is to be cut thousands of feet below surface, the machine must grasp the pipe and undertake the cut without visual supervision. It has been found, however, that frequently the grasping arms do not force the length of pipe equally against all the retaining pads such that the device is somewhat skewed relative to the pipe. As a consequence of the skewed pipe, the machine may shake during the cutting process or realign itself against the pipe causing the cutting wire to snap.
The loop of wire that is the cutting element cannot be kept taut as it is applied against the surface but must be allowed to bend such that the longitudinal strength limitations of the wire are not exceeded. Currently it is the practice to provide a space consuming serpentine wire take-up assembly that will retain tension on the wire and allow the loop to be expanded as the cutting device is applied against the pipe. The serpentine wire take-up significantly enlarges the size of the machine and thereby makes it more cumbersome to handle.
It would be desirable to provide an improved machine that overcomes or reduces some of the forgoing problems. Specifically, it would be desirable to have a machine suitable for cutting a wide variety of sizes of pipe diameters such that a fewer number of machines are needed to cut all the sizes of submerged pipe that are available. It would also be desirable to provide a wire cutting machine that will more accurately grasp a length of pipe without requiring visual supervision and that does not require a serpentine wire take-up to prevent breakage of the wire.
The surrounding water in which the machine operates also causes resistance to movement and thereby reduces its efficiency. A major portion of the resistance generated occurs as a result of the rotation of the various wheels around which the loop of wire is moved. Even a wheel with a smooth surface will apply centrifugal forces to the surrounding water thereby reducing the efficiency of the machine.
One of the wheels is a drive wheel which applies force to the cutting wire causing it to rotate and cut the metal of a pipe. The drive wheel must therefore have a surface that contacts the cutting wire and has a sufficiently high coefficient of friction to apply the force to the wire needed to cut the metal of the pipe. The wheels of existing wire saws, excluding the drive wheels, are mounted on a shaft that extends between parallel plates, one on each side of the wheel such that an annular insert around the wheel cannot be replaced in the field.
Existing wire saws provide a strip of rubberized material that is bonded into an annular groove around the circumference of the wheel for engaging the surface of the wire and applying force to the wire causing it to rotate. The rubberized material within the groove however is worn away rapidly as the saw is used and therefore must be replaced often. Furthermore, since the rubberized material is in the form of a strip, the deterioration of the material occurs most rapidly where the ends of the strip meet each other at one location around the circumference of the wheel. It would be desirable to provide a resistive surface for a drive wheel that is more resistant to deterioration and does not require assembly that leaves a junction that connects two ends of a strip. It is also desirable that the material that forms the resistive surface be easily replaceable so that the machine can be serviced in the field.
To a lesser extent, the surfaces of the various guide wheels that also engage the cutting wire of a wire saw must have a degree of flexibility so as to minimize damage to the cutting wire as it moves around the guide wheel, and it is common therefore to provide a rubberized insert that fits within the groove of each guide wheel. The softer material in the groove of the guide wheel that engages the cutting wire must also be readily replaceable. It would be desirable to provide wheels for the wire saw that can receive annular inserts that can be replaced while the machine is in the field.